A liquid crystal display apparatus is thin and lightweight and the number of its applications has been further increased in recent years as a substitute for a conventional cathode ray tube. A TN (Twisted Nematic) oriented liquid-crystal panel widely used at present has a small angle of visibility and a low response speed and its liquid-crystal element is the holding type. Therefore, the panel is inferior to a cathode ray tube because it leaves traces for animation display.
In general, in the TN oriented liquid-crystal panel, a phenomenon has been observed that a display pattern remains even after turning off a power supply. When the liquid-crystal backlight goes out after turning off the power supply, a phenomenon, that is, an “OFF afterimage” occurs, in which display is thinly left and seen due to reflected light of external light at a place where the external light is strong. This is caused by the fact that because the potential accumulated in a pixel electrode is not discharged and operations are completed while a TFT is in an open state, electric charge is not released even after the power supply is turned off and a voltage is continuously applied to liquid crystal.
Therefore, the TN oriented liquid-crystal panel has solved the problem by using the sequence of turning on all gates of the TFT to release electric charge when the power supply is turned off and then completing operations after electric charge is released.
By using OCB (Optically Compensated Bend) mode liquid crystal having bend orientation (for example, refer to Japanese Patent Laid-Open No. 61-116329), it is possible to sufficiently correspond to animation display and an increased screen size with a high speed response and a wide angle of visibility and provide a large-screen display having smaller thickness and power consumption than a cathode ray tube.
The orientation of OCB mode liquid crystal includes splay orientation and bend orientation. As shown in FIG. 8(a) the splay orientation denotes liquid crystal orientation in a state in which a voltage is not applied to OCB mode liquid crystal (non-display state) and the bend orientation denotes liquid crystal orientation which is kept in a display state by applying a transfer voltage to the OCB mode liquid crystal. Transition from the bend orientation to the splay orientation is made by bringing an applied voltage into zero or below a counter-voltage Vc (hereafter referred to as inverse transfer) In this case, the counter-voltage Vc denotes a voltage in which the energy of the splay orientation is substantially balanced with the energy of the bend orientation and the splay orientation becomes stable at the voltage or lower.
However, even in the case of the OCB mode liquid crystal, an OFF afterimage occurs due to the above residual electric charge. Moreover, an OFF afterimage occurs in a mode other than the above mode. Therefore, this problem is not solved by only turning on gates of TFTs. Specifically, unevenness has been recognized on a display face unless the process in which the bend orientation used for display of the OCB mode liquid crystal transitions to the splay orientation is uniform. Particularly, because an image ununiformly disappears from a display face depending on a display pattern, uncomfortable feeling occurs to a user.
Specifically, when the OCB mode liquid crystal transitions from the bend orientation to the splay orientation, it progresses in accordance with the following steps. First, when a voltage to be applied to the OCB mode liquid crystal becomes 0 V, the bend orientation becomes unstable and 180° twist occurs in all regions. In this case, the 180° twist denotes liquid crystal orientation in which the alignment direction of liquid crystal molecules is twisted between an upper substrate and a lower substrate and its twist angle is 180°. This orientation state is recognized as transparent bright yellow for example. This twist orientation state may be referred to as second splay orientation.
When no voltage is applied to the OCB mode liquid crystal, the splay orientation is more stable than the twist orientation state. Therefore, a splay orientation region remaining on a display face and/or a splay orientation region that incidentally occurs by foreign matter or protrusion on the display face as a core grow. Finally, the entire display face becomes the splay orientation and is stabilized. This splay orientation is, for example, transparent blue.
A problem is that a state in which the twist orientation (yellow) and splay orientation (blue) after turning off the power supply are mixed is seen as an uneven state on a display face ununiformly or depending on the pattern at the time of display.
In the case of the OCB liquid crystal, when the transition from the bend orientation to the splay orientation is ununiform, it takes time for the entire face of a liquid-crystal layer to change to the splay orientation after turning off the power supply. FIG. 19 is a time chart showing operations when turning off the power supply of a liquid crystal display apparatus using conventional OCB mode liquid crystal (hereafter referred to as power-supply OFF sequence). According to the power-supply OFF sequence shown in FIG. 19, a backlight is turned off and at the same time, a voltage to be applied to the liquid crystal layer is turned off at the timing of turning off a liquid-crystal driving power supply.
According to this power-supply OFF sequence, a portion to be quickly changed to the splay orientation and a portion to be slowly changed to the splay orientation are produced when changing from the bend orientation to the splay orientation among display screens after turning off the power supply because the applied voltage of each portion of the liquid crystal layer depends on image display. Therefore, in a predetermined time until completely changing to the splay orientation after turning off the power supply, a portion of the liquid crystal layer is already changed to the splay orientation but an orientation state still between the bend orientation and the splay orientation (hereafter referred to as second splay orientation) occurs in another portion. In this case, when external light is strong, the difference between orientation states of various portions of the liquid crystal layer is seen as unevenness even if turning off the backlight. For example, at room temperature, approx. 5 sec is required for the change of all liquid crystal layers to the splay orientation.
Moreover, when turning on the power supply again, in a time until completely changing to the splay orientation after turning off the power supply, a long transfer driving period of changing to the bend orientation is necessary when the power supply is turned on and an excessive time is required until an image is displayed after turning on the power supply.
FIG. 20 shows a time chart showing operations of a liquid crystal display apparatus using OCB mode liquid crystal when turning on a power supply. When the power supply is turned on at the time t0, a factor for splay orientation to be disarranged is added to a liquid crystal layer due to wraparound from various routes of a circuit immediately after the time t0. To correct the disarrangement of the splay orientation, 0 V is applied to the liquid crystal layer in the period from the time t0 to the time t1. Then, after the liquid crystal layer becomes uniform splay orientation, a transfer voltage for driving transfer of the liquid crystal layer is applied in the period from the time t1 to the time t2. After transfer driving is completed at the time t2, a voltage of displaying an image on a display face is applied to the liquid crystal layer.
In this case, when the power supply is turned on again, in the time until completely changing to the splay orientation after turning off the power supply as the above mentioned, disarrangement in second splay orientation is added to the disarrangement of the splay orientation that occurs when the power supply is turned on. Therefore, a long time is required from the time t0 to the time t1. For example, the period from t0 to t1 when the power supply is turned on in the state where the second splay orientation is not present is approx. 0.2 sec while the period from the time t0 to the time t1 when the power supply is turned on at the presence of the second splay orientation requires approx. 0.4 sec. Thus, when the second splay orientation is present, the period until an image is displayed after turning on the power supply is increased.